1. Field of the Invention
This invention relates to an apparatus for recording information by scanning a recording medium by a light beam condensed into a spot-like shape, and in particular, to an optical information recording apparatus provided with focusing and/or tracking servo means for the light beam.
2. Related Background Art
Among the forms of medium using light to record information thereon and to read the recorded information, are a disc-like form, a card-like form and a tape-like form. Among these, an optical information recording medium formed into a card-like shape (hereinafter referred to as the "optical card") has a great estimated demand as a medium of great recording capacity which is compact, and lightweight and convenient to carry.
Referring to FIG. 1 of the accompanying drawings which is a schematic plan view of such an optical card 101, the reference numeral 102 designates an information recording area, the reference numeral 103 denotes information tracks, the reference numerals 104 and 104' designate track selecting areas, and the reference numeral 105 denotes the home position of a light beam spot.
On the optical card 101, information is optically detectably recorded as a record bit row (information tracks) with scanning the optical card by a light beam which is modulated in accordance with record information and concentrated into a minute spot. At that time, to record the information accurately without causing problems such as intersection between the information tracks, it is necessary to control the position of application of the light beam spot on the surface of the optical card in a direction perpendicular to the scanning direction (the lengthwise direction of the tracks) (auto tracking which will hereinafter be referred to as "AT"). It is also necessary to control the light beam in a direction perpendicular to the surface of the optical card (auto focusing which will hereinafter be referred to as "AF") to apply the light beam as a minute spot of stable size in spite of the bending or mechanical error of the optical card. Also, AT and AF are necessary during reproduction.
Referring now to FIG. 2 of the accompanying drawings which shows the construction of an apparatus for recording and reproducing information on the optical card, the reference numeral 106 designates a motor for driving the optical card 101 in the direction of the arrow parallel to the lengthwise direction of the tracks, the reference numeral 107 denotes a light source such as a semiconductor laser, the reference numeral 108 designates a collimator lens for collimating the light from the light source 107, the reference numeral 109 denotes a beam splitter, the reference numeral 110 designates an objective lens, the reference numeral 111 denotes a coil for tracking, the reference numeral 112 designates a coil for focusing, the reference numerals 113 and 114 denote condensing lenses, the reference numerals 115 and 116 designate photoelectric conversion elements, the reference numeral 117 denotes a tracking control circuit, and the reference numeral 118 designates a focusing control circuit. Electric current is sent to the coil 111 for tracking and the coil 112 for focusing by the commands from the control circuits 117 and 118 on the basis of a tracking signal and a focusing signal detected by the photoelectric conversion elements 115 and 116 to thereby move the objective lens 110 and accomplish AT and AF. The reference numeral 119 designates a system controller for controlling the recording-reproducing apparatus, and the reference numeral 120 denotes a group of various control signals output from the system controller. Other signals than the signals 120 are also output from the controller 119, but they are not shown. The reference numeral 122 designates a drive motor for moving an optical head in the direction of arrow u in FIG. 1.
The light from the light source 107 is collimated by the collimator lens 108, passes through the beam splitter 109 and thereafter is condensed on a recording track on the optical card 101 by the objective lens 110. The light reflected by the recording track is now transmitted through the beam splitter 109 and is divided into two light beams by the beam splitter 109, and the divided light beams are condensed on the tracking signal detecting photoelectric conversion element 115 and the focusing signal detecting photoelectric conversion element 116, respectively, by the condensing lenses 113 and 114, respectively. Signals obtained by the photoelectric conversion elements 115 and 116 are made into a tracking error signal and a focusing error signal by the tracking control circuit 117 and the focusing control circuit 118, respectively, and electric current is sent to the coil 111 for tracking and the coil 112 for focusing, whereby the objective lens 110 is moved to thereby accomplish AT and AF.
FIG. 3 of the accompanying drawings is a detailed diagram of the tracking control circuit 117.
In FIG. 3, the reference numeral 301 designates reflected light originating from the light source being reflected by the medium, the reference characters 115-A and 115-B denote photoelectric conversion elements divided into two by a dividing line extending in a direction corresponding to the lengthwise direction of the tracks for detecting a tracking error, and the reference numeral 207 designates a subtraction circuit for subtracting signals D and E which are signals from the photoelectric conversion elements 115-A and 115-B and for outputting a tracking error signal. The reference numeral 206 denotes an adder circuit for adding the signals D and E together, the reference numeral 203 designates a dividing circuit for dividing the output G of the subtraction circuit 207 by the output F of the adder circuit 206, the reference numeral 202 denotes a phase compensator circuit for achieving the stabilization of AT servo, the reference numeral 204 designates a driver for sending a drive current to the objective lens, and the reference numeral 205 denotes a tracking controller for receiving the signal 120 from the system controller. 119 and controlling the entire tracking control circuit 117.
FIGS. 4A-4D of the accompanying drawings are timing charts of the signals, at various positions of the FIG. 3 circuit during recording.
FIG. 4A shows a record information signal, FIG. 4B shows a variation in the power P of the light source modulated by said signal, FIG. 4C shows a variation in the voltages of tracking signals D and E obtained by the power of the light source via the photoelectric conversion elements 115-A and 115-B, and FIG. 4D shows a variation in the voltage of a signal F which is the addition signal of the tracking signals D and E.
By "1" and "0" of the record information, the power P of the light source is varied into two stages P.sub.H and P.sub.L, and setting is made so that when the power is P.sub.H, a pit is formed and when the power is P.sub.L, no pit is formed. At this time,. the signals D and E are also varied into two stages and therefore, if these signals D and E are only subtracted, the open loop gain of AT servo when the power of the light source is P.sub.H becomes P.sub.H /P.sub.L times as great as the open loop gain of AT servo when the power of the light source is P.sub.L. This has led to the problem that oscillation is liable to occur and the servo becomes unstable. Particularly in the case of an optical card in which the line speed is low and the zone of record information is near the zone of AT servo, the component of the record information mixes with the AT servo with a result that the objective lens is unnecessarily moved and thus, stable AT servo becomes impossible. Consequently, as an example for making the open loop gain of AT servo constant both when the power of the light source is P.sub.L and when the power of the light source is P.sub.H, there is a method using the dividing circuit 203 as shown in FIG. 3. That is, the output G of the subtraction circuit 207 is input to the numerator side input terminal Y of the dividing circuit 203 and the output signal F of the adder circuit 206 is input to the denominator side input terminal X of the dividing circuit 203. Thus, the variation by record modulation is offset, because even if the signals D and E are varied into V.sub.L and V.sub.H as shown in FIG. 4C, the signal F is likewise varied into V.sub.DL and V.sub.DH as shown in FIG. 4D, and the open loop gain of AT servo can always be made constant and it becomes possible to accomplish a stable AT servo operation.
This method is also applicable to AF servo. For example, a focusing signal is detected by using an anamorphic optical system such as a toric lens like the lens 114 of FIG. 2 and using, as the photoelectric conversion element 116, an element whose light-receiving surface is divided into four portions 116-A to 116-D as shown in FIG. 5. A spot condensed on this light-receiving surface by the lens 114 is circular when the light beam is focused on the medium, but when out-of-focus, the spot is deformed in conformity with the direction of the out-of-focus as indicated by the broken line and the dot-and-dash line in FIG. 5 of the accompanying drawings. Accordingly, the sum signal 0 of the outputs of portions 116-A and 116-C which are at diagonal positions and the sum signal P of the outputs of portions 116-B and 116-D are calculated by adders 130 and 131, respectively, and these are differentiated by a differential amplifier 132 to thereby obtain a focusing signal.
The basic construction of the optical card recording-reproducing apparatus as described above is disclosed, for example, in U.S. Pat. No. 4,912,697. Also, the dividing circuit of the focusing servo is described in Japanese Laid-Open Patent Application No. 52-134704.
However, the dividing circuit 203 in the above-mentioned example of the prior art requires complicated analog processing therein and therefore, is expensive and lacks precision. This has led to an increased expense of the servo circuit as a whole and to the disadvantage that the accuracy of the information recording is reduced.
On the other hand, an optical information recording-reproducing apparatus in which the gain of the focusing servo is changed over between the time of recording and the time of reproduction during which the intensity of the light beam applied to the medium differs is disclosed in Japanese Laid-Open Patent Application No. 52-80802. However, again in this apparatus, there has been the problem that during recording, the gain is always constant and particularly at a point of time when the intensity of the light beam varies, servo becomes unstable.